1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying method for purifying exhaust gas from an internal combustion engine of a motor vehicle or the like and, more particularly, to an exhaust gas purifying catalyst and an exhaust gas purifying method capable of efficiently removing nitrogen oxides (NOx) by reduction, from exhaust gas that contains an amount of oxygen in excess of the amount of oxygen required for complete oxidation of the reducing components of the exhaust gas, such as carbon monoxide (CO), hydrogen gas (H.sub.2), hydrocarbon (HC) and the like
2. Description of the Related Art
Many conventional motor vehicles employ three-way catalysts that purify exhaust gas by simultaneously oxidizing CO and HC and reducing NOx in exhaust gas at a theoretical air-fuel ratio (stoichiometric ratio). In a widely known three-way catalyst of this type, a heat-resistant base member formed of, for example, cordierite, carries thereon a porous support layer formed of .tau.-alumina, and the porous support layer supports catalytic noble metals such as platinum (Pt), rhodium (Rh) and the like.
Recently, carbon dioxide (CO.sub.2) in exhaust gas from internal combustion engines of motor vehicles and the like has become and issue in view of protection of global environments. A promising technology for reducing the amount of CO.sub.2 emission from an internal combustion engine is a lean burn system, in which combustion is performed at a lean air-fuel ratio with an excessive amount of oxygen. The lean burn system reduces fuel consumption due to improved fuel efficiency and, therefore, reduces the amount of CO.sub.2 production by combustion.
Since the conventional three-way catalysts achieve simultaneous oxidation of CO and HC and reduction of NOx in exhaust gas when the air-fuel ratio is substantially the stoichiometric ratio, the conventional three-way catalysts fail to sufficiently remove NOx by reduction in lean-burnt exhaust gas, which contains an excessive amount of oxygen. Therefore, there has been a need for development of a catalyst and an exhaust gas purifying system capable of removing NOx even in an excessive-oxygen atmosphere.
The present applicant proposed in, for example, Japanese Patent Application Laid-Open No. Hei 5-317652, an exhaust gas purifying catalyst in which an alkaline earth metal, such as barium (Ba) or the like, and platinum (Pt) are supported by a porous support formed of alumina or the like. Using the exhaust gas purifying catalyst, NOx can be efficiently removed from lean-burnt exhaust gas from a lean burn system if the air-fuel ratio is controlled so that the air-fuel ratio shifts from a lean side to a stoichiometric/rich side in a pulsed manner. NOx is adsorbed by the alkaline earth metal (NOx adsorbent) on the lean side, and reacts with reducing components, such as HC, CO and the like, on the stoichiometric/rich side.
It has been found that the aforementioned NOx-removing process using the exhaust gas purifying catalyst proceeds in three steps: the first step in which NO in exhaust gas is oxidized into NOx; the second step in which NOx is adsorbed by the NOx adsorbent; and the third step in which NOx released from the NOx adsorbent is reduced on the catalyst.
However, in the conventional exhaust gas purifying catalysts, particle growth of platinum (Pt) occurs in a lean atmosphere, thereby reducing the number of catalytic reaction points. Therefore, the reactivity in the first and third steps inconveniently decrease.
Rhodium (Rh) is known as a catalytic noble metal that has a lesser tendency to undergo particle growth in a lean atmosphere. However, the oxidizing capacity of Rh is considerably lower than that of Pt. Use of a combination of Pt and Rh may be considered. It is known that co-presence of Pt and Rh reduces the particle grow of Pt.
With regard to combined use of Pt and Rh, it has been found that as the amount of Rh contained increases, the oxidizing capacity of Pt decreases, probably because Rh covers Pt surfaces. Therefore, as the Rh content increases, the reactivity of oxidation of NO into NOx in the first step decreases and the NOx-adsorbing rate in the second step also decreases. Another problem with Rh is low compatibility with NOx adsorbents. Co-presence of Rh and a NOx adsorbent results in insufficient performance of the NOx adsorbent and Rh.
In addition, sulfur (S) components contained in fuel are oxidized into SO.sub.2, which is further oxidized on the catalyst into sulfates. If sulfates react with the NOx adsorbent, the NOx adsorbing capacity of the NOx adsorbent is lost, thereby impeding the removal of NOx by reduction. This undesired phenomenon is generally termed sulfur-poisoning of NOx adsorbent.